Batteries
A battery, as used herein, refers to a cell having two electrodes, an anode and a cathode, separated by an electrolyte. The cell may be an electrochemical cell. Materials in the electrodes may be both electronically and chemically active. The anode may be a chemical reductant and the cathode may be a chemical oxidant. Thus, both the anode and the cathode may be able to gain and lose ions, typically the same ion, which is referred to as a ‘working ion’ of the battery. The electrolyte may be chemically active while being electronically passive. The chemical activity of the electrolyte is exhibited by the ability to gain and lose ions, which are typically the working ion. The electrolyte is generally an electronic insulator, and may not promote the movement of electrons within the battery.
The battery may operate via a chemical reaction between the two electrodes that has an electronic and an ionic component, and is hence called an ‘electrochemical reaction’. The electrolyte conducts the working ion inside the cell and, as an electronic insulator, enables electrons involved in the reaction to pass through an external circuit.
When a liquid or polymer electrolyte is used in a battery, a separator that remains an electronic insulator on contact with the two electrodes may be used to keep the two electrodes from electronically contacting each other inside the cell. The separator may be permeated by the liquid or polymer electrolyte to allow ionic conduction between the two electrodes. In some battery implementations, a solid electrolyte may be used as the separator. Solid electrolytes may be used alone or with a liquid or polymer electrolyte contacting one or both electrodes. Solid electrolytes may also function as a separator, such that a separate separator is not needed.
Batteries are often named after the working ion. For instance, lithium ion (Li+) is the working ion in a lithium-ion (Li-ion) battery. Sodium ion (Na+) is the working ion in a sodium-ion (Na-ion) battery. Li-ion batteries are commonly used in electronic devices, power tools, and electric vehicles. A Li-ion battery is assembled in a discharged state to enable preparation of a high-voltage cathode and an anode free of metallic lithium, while a flammable organic-liquid electrolyte may be used. Incremental improvements in Li-ion batteries have been obtained by the fabrication of complex electrode morphologies, but the carbon anode has limited capacity and may be plated by metallic lithium under an excessively high rate of charge. In addition, oxygen may be lost from a layered-oxide cathode if the cell is overcharged. Managing a large stack of cells over many charge and discharge cycles may increase the cost of a large, multi-cell battery, such as those used in electric vehicles. Moreover, the ability to increase the volumetric capacity of batteries, which is applicable for portable batteries, among other applications, has been limited. Finally, organic-liquid electrolytes suitable for most Li-ion batteries are flammable, posing safety risks, particularly if the batteries form dendrites or are damaged in some way that allows electronic contact between the cathode and the anode within the battery.
Supercapacitors
Supercapacitors utilize the capacitances of an electronic double layer at an electrode-electrolyte interface where positive and negative charges are separated by only atomic dimensions. Moreover, if the electrolyte has a large dielectric constant as a result of the presence of electric dipoles, the capacitance can be increased even further. The supercapacitor stores electric power as static electric charge rather than as chemical energy even where the supercapacitor may contain a faradaic component of electrical-energy storage.